1. Field of the Invention
The present invention relates, generally to bi-directional overrunning clutch assemblies and, more specifically, to a bi-directional clutch having a selectively controllable engagement assembly that is driven by an actuator that is operatively controlled by a momentary actuating force.
2. Description of the Related Art
Generally speaking, land vehicles require three basic components. These components include a power plant (such as an internal combustion engine), a power train and wheels. The power train's main component is typically referred to as the “transmission.” Engine torque and speed are converted in the transmission in accordance with the tractive-power demand of the vehicle. Transmissions include one or more gear sets, which may include an inner gear, intermediate planet or pinion gears that are supported by their carriers, and outer ring gears. Various components of the gear sets are held or powered to change the gear ratios in the transmission. In addition to such planetary gear sets, driveline components may further include multi-disc friction devices that are employed as clutches or brakes. The multi-disc pack clutch is a friction device that is commonly employed as a holding mechanism in a transmission, transfer case or differential or the like. In addition, multi-disc friction devices also find use in industrial applications, such as wet brakes, for example, to brake the wheels on earth-moving equipment.
The multi-disc pack clutch or brake assembly has a clutch sub-assembly including a set of plates and a set of friction discs that are interleaved between one another. The plates and friction discs are bathed in a continual flow of lubricant and in “open pack” operation normally turn past one another without contact. The clutch or brake assembly also typically includes a piston. When a component of a gear set is to be held, as for example during a particular gear range, a piston is actuated so as to cause the plates and friction discs to come in contact with respect to one another. In certain applications, it is known to employ several multi-disc pack clutch devices in combination to establish different drive connections throughout the transmission, transfer case, or differential to provide various gear ratios in operation, or to brake a component.
When the discs are not engaged, there often remains a differential rotational speed of the drive and driven members which the clutch or brake bridges. Relative rotation between the friction discs and the plates during open-pack mode creates drag. This condition results in parasitic energy losses, reduces the efficiency of the transmission, transfer case or differential, and ultimately results in lower fuel efficiency.
In addition to multiple friction devices, one-way clutches are frequently employed in transmissions, transfer cases, and differentials to selectively transmit torque in one rotational direction, but not in the opposite rotational direction. To this end, one-way clutches typically include an inner race, an outer race, and an engagement mechanism disposed therebetween. The engagement mechanism is operable to lock the inner and outer races together thereby transmitting torque in one relative direction. The engagement mechanism is further operable to allow freewheeling rotation between the inner and outer races in the opposite rotational direction. Engagement mechanisms commonly used in one-way clutches of the related art include pawls, sprags, and rollers. A cage, along with biasing members, such as springs, are also sometimes employed to retain the pawls, sprags, or rollers between the inner and outer races as well as to selectively assist in the change of operational modes between torque translation and freewheeling actuation of the clutch, depending on the direction of rotation between the inner and outer races.
As noted above, one-way clutches of this type have been employed in numerous applications in transmission, transfer cases, and differentials. For example, one-way clutches have been employed in conjunction with multiple friction clutches and planetary gear sets to effect low and reverse gear ratios in conventional transmissions. While this arrangement has worked well for its intended purpose, some disadvantages remain. For example, the friction clutch remains a source of significant parasitic losses due to inherent drag between the friction plates when the clutch is operating in “open pack” mode. Still, the clutch is necessary for providing the proper holding torque in low and reverse gears. Accordingly, there remains a need in the art for a mechanism that can provide the appropriate holding torque for both low and rear gears in the transmission and yet results in less parasitic losses which are presently attributable to the multiple plate friction clutch used for this purpose. In addition, there is a need in the art for a device that continues to perform the functions of the one-way clutch as described above, particularly where the output speed of the transmission exceeds the input speed resulting in engine compression braking.
One-way clutches have also been employed in transfer cases that provide full time, part time, and “on demand” four wheel drive (4WD) capabilities. In these situations, the one-way clutch is typically disposed between the primary driveline and the secondary driveline. When the primary drive line attempts to over speed the secondary drive line, as will occur, for example, where the rear wheel is supported on a slick surface, such as ice and is spinning and the front wheels are solidly supported, the one-way clutch engages and transfers torque to the slipping wheel. In this way, 4WD is achieved, but in this case, only under circumstances that require it.
The use of a one-way overrunning clutch to selectively provide drive torque to a secondary driveline upon primary wheel slip has not, however, become a popular solution to part time 4WD vehicle requirements because of one problem: the clutch remains disengaged or inactive when reverse gear is selected unless, of course, the secondary driveline attempts to over speed the primary driveline. Thus, in a situation frequently requiring 4WD, that is, when the vehicle may need to be rocked or simply backed over terrain, a 4WD configuration utilizing a one-way overrunning clutch will simply not provide 4WD operation. This is a significant drawback of this clutch configuration.
Partially in response to this problem, bi-directional overrunning clutches have been proposed in the related art for use in these circumstances. These bi-directional overrunning clutch assemblies typically employ an inner race, an outer race, and a plurality of rollers as the engagement mechanism disposed therebetween. The bi-directional overrunning clutches of the prior art are, for the most part, designed to be self-actuating. While they appear to present a solution to certain problems identified above they have not been widely employed in transmission, transfer cases, and differentials of the related art. These self-actuating bi-directional overrunning clutches are relatively mechanically complex and have certain physical limitations and drawbacks. One such drawback is that the existing bi-directional clutches have a large angular distance from the engagement in one rotational direction to the engagement in the opposite rotational direction. This makes for undesirable driving conditions by causing hard lockups when changing directions and also brings about a short mechanical life due to the severe inherent impact forces of the engagement. Additionally, the self-actuating bi-directional clutches known in the related art cannot be selectively engaged in an efficient manner or to optimize the vehicle power output in response to varying driving conditions.
Accordingly, selectively actuated bi-directional clutches have evolved that can be actuated and controlled in a manner to provide driving comfort and offer efficient operating modes for various driving conditions while eliminating the need for conventional multi-disc friction devices. These selectively actuated and controlled bi-directional clutches provide torque translation in either rotational direction and have been considered as a substitute for conventional multi-disc friction devices presently known in the related art in certain applications. However, there still exists room for further improvements in the manner in which these devices are selectively actuated.
Currently, these selectively actuated bi-directional clutches may employ either rollers that interact with narrowed disc surfaces or pawl and ratchet (i.e. teeth) engagement assemblies. Typically, a pawl-type engagement assembly is used where angular accuracy is required in the engagement. More specifically, the pawl-type selectively actuated bi-directional clutches are operatively controlled by actuating devices that rotate an actuator disc assembly in a manner to cause a set of engagement pawls to either engage or disengage an inner race to an outer race. In turn, the actuating discs are responsive to an electromagnetic, hydraulic, or other force generating medium to move the pawls between their operative modes. Since these clutches are bi-directional, there are generally two sets of engagement pawls and two actuator discs. While this arrangement works well, it requires that the actuating force that rotates the actuator discs remain constantly active to hold the respective actuator discs in position. This has certain drawbacks and can cause particular operative problems.
For example, if the actuating discs of the current type of bi-directional clutch are each controlled by a solenoid, then electrical power must be applied and constantly maintained to each of the solenoids to first actuate and to then hold the discs in position. If the electrical power to the solenoid is not maintained, the actuator discs may move in an uncontrolled manner to the opposite engagement position or to some point in-between. In either case, an uncontrolled movement could be physically devastating to the clutch and its components and possibly other parts of the drivetrain. At a minimum, an uncontrolled actuation of one, or both of the actuating discs would cause undesirable actions within the transfer case, or transmission, or other device in which the clutch may be installed. This is problematic when one considers that any minor drop off in electrical power, or the occurrence of some type of momentary or prolonged electrical break would cause the actuator discs to be uncontrolled and free to randomly move. Likewise, hydraulically controlled actuator discs suffer the same problem if a fluctuation, or drop off in hydraulic force where to occur.
A secondary consideration of selectively activated bi-directional clutches is that maintaining the electrical, or hydraulic pressure to the actuator requires a constant expenditure of energy. Thus, there is a constant load to the electrical or hydraulic system to provide the constant engagement force. This is a power requirement that must be accounted for in the design of the electrical or hydraulic support system, which ultimately adds to the size and cost of these systems. Therefore, there exists a need in the art for a selectively actuated and controlled bi-directional clutch that utilizes an actuator which holds the actuating discs in position and does not require a constantly applied force to maintain the actuator discs in their desired positions.